Total disc implant

ABSTRACT

A total disc implant (TDI) is provided for total replacement of a spinal disc or discs in a human patient or other mammal, wherein the TDI is designed to maintain a substantially full range of natural motion (ROM) following implantation. The TDI generally comprises, in one preferred form, upper and lower end plates for affixation to adjacent vertebral bodies, with an intervening insert disposed therebetween. The end plates each include elongated part-cylindrical surfaces oriented generally perpendicular to each other, with one of said surfaces extending in an anterior-posterior direction and the other extending in a medial-lateral direction. The intervening insert defines concave upper and lower part-cylindrical seats oriented for respectively engaging these part-cylindrical surfaces, wherein these part-cylindrical seats are defined by offset radii to include a somewhat flattened central base region merging smoothly with upwardly curving radiused sides.

BACKGROUND OF THE INVENTION

This application claims the benefit of U.S. Provisional Application60/434,092, filed Dec. 17, 2002.

A novel motion preserving total disc replacement implant (TDI) isprovided. The TDI is particularly designed for implantation into a humanpatient or other mammal, into the inter-vertebral space between adjacentspinal discs or vertebrae, as a prosthetic replacement for one or moresurgically removed discs. The TDI beneficially provides a substantiallyfull and natural post-operative range of motion (ROM).

In the preferred form, the components of the TDI of the presentinvention are formed from ceramic materials, or biocompatible metals, ora combination thereof, with preferred ultra-low wear ceramic-ceramic orceramic-metal articulatory components and materials being described incopending U.S. Ser. No. 10/171,376, filed Jun. 13, 2002, and entitledMETAL-CERAMIC COMPOSITE ARTICULATION, which is incorporated by referenceherein. Such ultra-low wear bearing material or materials have shownimpressive mechanical and tribological properties for hip articulations,and may be used in the TDI of the present invention thereby avoiding theproblems and disadvantages associated with prior art concepts usingmetal end plates articulating with a conventional high molecular weightpolyethylene (PE) insert.

Spinal disc herniation and the often resultant symptoms of intractablepain, weakness, sensory loss, incontinence and progressive arthritis areamong the most common debilitating conditions affecting mankind. If apatient's condition does not improve after conservative treatment, andif clear physical evidence of nerve root or spinal cord compression isapparent, and if correlating radiographic studies (i.e., magneticresonance imaging (MRI) or X-ray computer tomography (CT) imaging ormyelography) confirm the condition, discectomy, or surgical removal ofthe affected disc is often resorted to. In the United States in 1985,over 250,000 such operations were performed in the lumbar spine andcervical spine.

Statistics suggest that present surgical techniques are likely to resultin short-term relief, but do not prevent the progressive deteriorationof the patient's condition in the long run. Through better pre-operativeprocedures and diagnostic studies, long-term patient results haveimproved somewhat. But it has become clear that unless the removed discis replaced or the spine is otherwise properly supported, furtherdegeneration of the patient's condition will almost certainly occur.

In the mid-1950's and 60's, Cloward and Smith & Robinson popularizedanterior surgical approaches to the cervical spine for the treatment ofcervical degenerative disc disease and related disorders of thevertebrae, spinal cord and nerve root; these surgeries involved discremoval followed by interbody fusion with a bone graft. It was noted byRobinson ^(i) that after surgical fusion, osteophyte (bone spur)re-absorption at the fused segment might take place. However, it hasbecome increasingly apparent that unfused vertebral segments at thelevels above and below the fused segment degenerate at accelerated ratesas a direct result of this fusion. This has led some surgeons to performdiscectomy alone, without fusion, by a posterior approach in the neck ofsome patients. However, as has occurred in surgeries involving the lowerback where discectomy without fusion is more common as the initialtreatment for disc herniation syndromes, progressive degeneration at thelevel of disc excision is the rule rather than the exception.

^(i) R A Robinson, “The Results of Anterior Interbody Fusion of theCervical Spine”, J. Bone Joint Surg., 440A: 1569-1586, 1962.

Similarly, in addition to the problems created by disc herniation,traumatic, malignant, infectious and degenerative syndromes of the spinealso involve fusion of spine segments. Other procedures include bonegrafts and metallic rods, hooks, plates and screws being appended to thepatient's anatomy; often they are rigidly and internally fixed. Noneprovide for a patient's return to near-normal functioning. Though theseprocedures may address the symptoms in the short-term, they can resultin progressive degeneration of discs at adjacent levels in the longerterm. This is due to the adjacent discs attempting to compensate for thelack of motion of the fused segment. In fact, it is now well recognizedthat premature degenerative disc disease at the level above and belowthe excised disc can and does occur. Hence, motion preserving total discreplacements are a promising alternative to spine fusion devices. Thisnext generation of spinal implants in fact, mirror the progression inother articulating joints such as hips and knees: from arthrodesis toarthroplasty.

While long term clinical data are unavailable, the current generation ofarticulating disc implants typically have metal end plates with acompliant articulating, typically high density polyethylene (PE) insertbetween them. Compliant inserts are used to enable low frictionarticulation and also to enable resilient cushioning under load,although no clinical proof exists that shock absorption is necessary.The long history of similar metal/PE articulations for hip and kneeprotheses indicates that in the long term, PE wear particles are one ofthe principal causes of implant failures. In hip implants for example,the ultra-high molecular weight polyethylene (PE) particles are releasedover time from the acetabular liner^(ii, iii, iv). This wear debris isreleased into the peri-implant tissue and elicits a deleterious biologicreaction, incorporating foreign-body giant cell and macrophage cellresponses leading to bone resorption, and eventual implant failure. As aconsequence, alternate rigid-on-rigid bearing materials such asalumina-on-alumina ceramic, metal-on-metal, and the recentcobalt-chromium (CoCr) alloy-heavily cross-linked PE have beenintroduced.

^(ii) Callaway G. H, Flynn W., Ranawat C. S., and Sculco T. P., J.Arthroplasty, 10, No.6:855-859, 1995. ^(iii) Higuchi F., Shiba N., InoueA. and Wakabe I., J. Arthroplasty, 10, No.6: 851-854, 1995. ^(iv)Kirkler S. and Schtzker J., J. Arthroplasty, 10, No.6: 860-862, 1995.

It is instructive to follow the evolution of knee joint implants sincethey are kinematically analogous to intervertebral discs: they have asimilar range of complex motion including sliding in theanterior-posterior (A-P) direction, rotation and bending in themedial-lateral (M-L) direction, and combinations thereof. Early designshad unacceptable failures due to aseptic loosening resulting from poorconformity leading to instability, high contact stresses, and high PEinsert wear. Despite lower loads on the knee joint, backside wear of thePE tibial insert resulting from sliding motion was a major cause forconcern^(v). Implant stability was found to be a function of how wellthe tibial component was fixed. Extensive retrieval analysis of kneeimplants has indicated that proper fit, fixation and initial stabilityof the tibial component was critical to achieving clinical success.Stated alternatively, development of an appropriate ingrowth surface toachieve consistent bony fixation over large cancellous regions wascritical to implant success^(vi, vii). More recent knee implant designshave included mobile bearing platforms that allow rotation and A-Ptranslation. The articulations have improved conformity and kinematics,which lead to reduced contact stresses. However new problems arise:bearings without stops can dislocate or spin-out, and bearings withstops can wear as they abut against the mechanical stops that preventdislocation.^(viii)

^(v) Blunn G W, Walker P S, Joshi A, Hardinge K. The dominance of cyclicsliding in producing wear in total knee replacements. Clin Orthop. 1991;273:253-260. ^(vi) Bloebaum R D, Mihalopoulous N L, Jensen J W and DorrL D, “Postmortem analysis of bone ingrowth into porous acetabularcomponents”, JBJS, Vol. 79-A, No. 7, 1013, July 1997. ^(vii) Bloebaum RD, Bachus K A, Jensen J W, Scott D F, and Hofmann A A, “Porous coatedmetal-backed patellar components in total knee replacements”, JBJS, Vol.80-A, no.4, 518, April 1998. ^(viii) Bert J M, “Dislocation/subluxationof meniscal bearing elements after New Jersey Low-Contact Stress totalknee arthroplasty”. Clin Orthop. 1990; 254:211-215.

Diagnostic imaging using radiography or MRI is commonly used to assessthe presence of spinal disease, determine range of motion or evaluatethe patients progress in healing post surgical treatment^(ix, x). Thepresent generation of total disc replacements use metal end plates whichpresent problems with imaging MRI or in X-Ray-CT imaging, due to thepresence of halos and other artifacts.

^(ix) Huang R, Girardi F, Cammisa F Jr, and Marnay T, “Long termflexion-extension range of motion of the Prodics-I”, Proc. 17^(th) NASSAnn. Mtg., The Spine Journal, 2/93S, 2002. ^(x) McAfee P C, Cunningham BW, Devine J G, Williams E, and Yu-Yahiro J, “Classification ofheterotopic ossification in artificial disc replacement”, Proc. 17^(th)NASS Ann. Mtg., The Spine Journal, 2/94S, 2002.

Finally, an important requirement for total disc implants is that thearticulating disc does not protrude or impinge on the spinal cord ornerve roots, which is a concern with compliant materials.^(xi) As hasbeen well established from other articulating joints such as the knee,PE inserts can suffer damage from several modes: creep, pitting,scratching, burnishing, abrasion, delamination and embeddedparticulates. While there is debate over whether creep or wear is themain cause of dimensional changes in PE inserts^(xii, xiii), there islittle doubt that damage to PE can and does occur over the long term.

^(xi) Ledet E, Drisio D, Tymeson M, Anderson L, Kallakury B, Sheehan C,and Sachs B, “The Raymedica PDN prosthetic disc nucleus device in thebaboon lumbar spine”, Proc. 17^(th) NASS Ann. Mtg., The Spine Journal,2/94S, 2002. ^(xii) Wright T M, Hood R W and Burstein A H, “Analysis ofMaterial failures”, Orthop. Clin. North Am., 13, 33, (1982). ^(xiii)Wroblewski B M, “Wear and loosening of the socket in the Charnleylow-friction arthroplasty,” Orthop. Clin. North Am., 19, 627, 1988.

Thus, there is a need to develop an alternative to the presentgeneration of disc implants, typically with respect to those designedfrom metal/PE articulations, and to overcome a number of potentialdrawbacks:

-   -   [a] Long term wear of the articulating PE “disc”, especially in        cases where small bony fragments are entrapped between the        articulating surfaces,    -   [b] Osteolysis and subsequent aseptic loosening and instability        of the implant as a result of the PE wear debris,    -   [c] Protrusion of the disc from the disc space due to creep or        fatigue related gradual changes in dimensional characteristics,        and    -   [d] Difficulty of diagnostic imaging the intervertebral region        because of electromagnetic artifacts, halos and radiographic        shadows associated with the metal end plates.

The proposed TDI design of the present invention is geometricallyconfigured to accommodate a substantially full and natural range ofmotion, and, in the preferred form, is constructed from an alternateultra-low wear bearing material that restores anatomic function avoidsall the drawbacks of current artificial disc designs.

SUMMARY OF THE INVENTION

In accordance with the invention, a total disc implant (TDI) is providedfor total replacement of a spinal disc or discs in a human patient orother mammal, wherein the TDI is designed to maintain a substantiallyfull range of natural motion (ROM) following implantation. In generally,the TDI comprises upper and lower end plates for affixation to adjacentvertebral bodies, wherein this pair of end plates are adapted foraccommodating a substantially full and natural range ofanterior-posterior (A-P) rotation or flexion, medial-lateral (M-L)rotation or flexion, and axial rotation.

In one preferred form, the TDI generally comprises the upper and lowerend plates for affixation to adjacent vertebral bodies, in combinationwith an intervening insert disposed therebetween. The upper and lowerend plates include elongated and generally convex part-cylindricalsurfaces oriented generally perpendicular to each other, with one ofsaid surfaces extending in an anterior-posterior direction and otherextending in a medial-lateral direction. The intervening insert definesconcave upper and lower part-cylindrical seats oriented generallyperpendicular to each other for respectively engaging thesepart-cylindrical surfaces, but wherein at least one and preferably bothof these part-cylindrical seats are defined by offset radii to include asomewhat flattened central base region merging smoothly with upwardlycurving radiused sides. With this geometry, the TDI accommodates asubstantially full and natural range of motion, includinganterior-posterior flexion, medial-lateral extension, and a limitedrange of axial rotation.

In an alternative form, an elongated and generally convexpart-cylindrical surface is formed on one of the upper and lower endplates, and a generally concave part-cylindrical seat defined preferablyby offset radii is formed on the other of the two end plates. When theupper and lower end plates are suitably affixed to adjacent vertebralbodies, the part-cylindrical convex surface is retained in articulatingengagement with the part-cylindrical concave seat, in a mannerpermitting a substantially full and natural range of motion, includinganterior-posterior flexion, medial-lateral extension, and a limitedrange of axial rotation.

Preferred materials include ceramic, with a most preferred materialbeing sintered silicon nitride (Si₃N₄), for the upper and lower endplates and insert, or a biocompatible metal such as titanium orcobalt-chrome alloy, or a combination of such ceramic and metalmaterials. Preferred ceramic materials for use in a ceramic-ceramic or aceramic-metal articulation interface are disclosed in copending U.S.Ser. No. 10/171,376, filed Jun. 13, 2002, which is incorporated byreference herein.

Other features and advantages of the present invention will become moreapparent from the following detailed description, taken in conjunctionwith the accompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the invention. In such drawings:

FIG. 1 is an exploded top perspective view showing a total disc implantconstructed in accordance with one preferred form of the presentinvention, and illustrating upper and lower end plates with an insertpositioned therebetween;

FIG. 2 is a side elevation view of the total disc implant depicted inFIG. 1;

FIG. 3 is an enlarged side elevation view of the insert;

FIG. 4 is a top plan view of the total disc implant of FIG. 1, showingaxial rotation;

FIG. 5 is an anterior-posterior or sagital sectional view of the totaldisc implant of FIG. 1, showing anterior-posterior articulation;

FIG. 6 is a medial-lateral or coronal sectional view of the total discimplant of FIG. 1, showing medial-lateral articulation;

FIG. 7 is an exploded top perspective view showing a total disc implantconstructed in accordance with an alternative preferred form of thepresent invention;

FIG. 8 is an exploded bottom perspective view of the total disc implantof FIG. 7;

FIG. 9 is a side elevation view of the total disc implant depicted inFIG. 7;

FIG. 10 is an anterior-posterior or sagital sectional view of the totaldisc implant of FIG. 7, showing anterior-posterior articulation;

FIG. 11 is a medial-lateral or coronal sectional view of the total discimplant of FIG. 7, showing medial-lateral articulation;

FIG. 12 is a top plan view of the total disc implant of FIG. 7, showingaxial rotation;

FIG. 13 is a pair of photomicrographs comparing porosity and pore sizebetween a preferred cancellous structured ceramic material for use informing one or more portions of the total disc implant, with naturaltrabecular bone structure of a human lumbar vertebral body;

FIG. 14 is a radiograph showing the preferred cancellous structuredceramic material implanted a condylar bone of a sheep;

FIG. 15 is a back scattered electron (BSE) microscope image showing newbone ingrowth into the preferred cancellous structured ceramic material,and apposition along the host bone/implant interface;

FIG. 16 is an exploded top perspective view showing a total disc implantconstructed in accordance with a further alternative preferred form ofthe present invention;

FIG. 17 is a side elevation view of the total disc implant embodimentdepicted in FIG. 16;

FIG. 18 is an inverted side elevation view of an upper component of thetotal disc implant embodiment of FIG. 16;

FIG. 19 is a top plan view of the total disc implant embodiment of FIG.16, showing axial rotation;

FIG. 20 is an anterior-posterior or sagital sectional view of the totaldisc implant embodiment of FIG. 16, showing anterior-posteriorarticulation; and

FIG. 21 is a medial-lateral or coronal sectional view of the total discimplant embodiment of FIG. 16, showing medial-lateral articulation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The TDI design of the present invention is based on the principles ofmaintaining spine anatomy, restoring function by preserving segmentalmotion, providing immediate stability, withstanding spine loads safely,and providing rapid osteo-integration between implant/host bone.

FIGS. 1 and 2 show the proposed TDI design for lumbar spine. The designfeatures an upper end plate 10 and a lower end plate 12 formedrespectively with upper and lower surfaces that engage with the adjacentvertebral bodies (not shown). Each end plate 10, 12 includes a solid rim14 substantially circumscribing the respective upper and lower surfaceto rest on the cortex of the adjacent vertebral body. Fixation elementssuch as fins, teeth or pins 16 protrude axially from the respectiveupper and lower surfaces of the end plates 10, 12 to provide anchoringand immediate stability with the adjacent vertebral bodies. These upperand lower surfaces include or are surface-coated each to define a porousin growth surface 18 to permit and accommodate rapid bone in-growth andosteo-integration for long term stability. A variety of suitable boneingrowth coatings and materials are known to persons skilled in the art.In addition, while these in-growth surfaces are depicted with agenerally planar configuration in FIGS. 1 and 2, alternative geometriesparticularly, such as a convexly contoured or domed configuration formore optimal and extended surface area contact with adjacent porous orcancellous interior structures of prepared adjacent vertebral bodies,will be apparent to persons skilled in the art.

The anterior-posterior (A-P) and medial-lateral (M-L) dimensions of theupper and lower end plates 10, 12 are chosen to suit typicallumbar/cervical spinal body dimensions, such as an A-P dimension ofabout 20-25 mm and a M-L dimension of about 28-35 mm as viewed in theillustrative drawings. The illustrative end plates 10, 12 furtherinclude an anterior to posterior lordotic taper to better restore thenatural curvature of spine, as viewed in FIG. 2 which shows each endplate 10, 12 with a tapered thickness that increases in the anterior toposterior direction. As viewed in FIG. 2, upon assembly of the totaldisc implant (as will be described in more detail), this lordotic tapermay provide a posterior spacing between the end plates 10, 12 of about 8mm, with the upper and lower surfaces of the end plates 10, 12 taperingforwardly in the anterior direction at a diverging angle of about 6degrees.

The articulating lower surface of the upper end plate 10, and thearticulating upper surface of the lower end plate 12 each include aunique contour that permits a substantially normal range flexion in theA-P direction in combination with extension in the M-L direction, whileadditionally accommodating a limited range of axial rotation. Thesearticulating surfaces of the upper and lower end plates 10, 12respectively engage and articulate with an intervening insert 20 havinguniquely contoured upper and lower surfaces.

More particularly, the articulating lower surface of the upper end plate10 comprises a part-cylindrical, downwardly convex elongated bearingcomponent or strip 22 defining a bearing surface extending generally inthe M-L direction. The articulating upper surface of the lower end plate12 comprises a similarly sized and shaped, part-cylindrical and upwardlyconvex elongated bearing component or strip 24 oriented to define abearing surface extending generally in the A-P direction. Thus, the twobearing strips 22, 24 are oriented generally on orthogonal axes relativeto each other.

The insert 20 is captured between these bearing strips 22, 24, andincludes generally part-cylindrical recessed bearing seats 26 and 28formed respective in the upper and lower sides thereof, generally onmutually orthogonal axis, for respective reception and bearingengagement with the part-cylindrical bearing strips 22, 24. Accordingly,the articulating geometry between the upper bearing strip 22 on theupper end plate 10, with the upper bearing seat 26 on the insert 20,accommodates A-P rotation or flexion (as viewed in FIG. 5), with apreferred range of A-P flexion on the order of about 12-15°. In asimilar manner, the articulating geometry between the lower bearingstrip 24 on the lower end plate 12, with the lower bearing seat 28 onthe insert 20, accommodates M-L rotation or extension (as viewed in FIG.6), with a preferred range of M-L extension on the order of about 12-15°of lateral bending. While the illustrative drawings depict a relativelysmall insert 20 (in plan view, relative to the size of the end plates10, 12, persons skilled in the art will recognize and appreciate thatthe outer dimensions of the insert 20 can be selected and varied asdesired to suit a specific patient, and/or to reduce or eliminate therisk of insert dislocation or expulsion from between the end plates 10,12 during normal patient movement.

In accordance with one primary aspect of the invention, the recessedpart-cylindrical bearing seats 26, 28 formed on the insert 20 each havea part-cylindrical contour defined in cross sectional shape by offsetradii, as shown best in FIG. 3. In particular, each bearing seat 26, 28is defined by upwardly curving sides shown in the illustrative exampleof FIG. 3 to be formed on radii of about 7.3 mm, but wherein the centersof these radii are spaced apart or laterally offset by a small increment(0.7 mm in the illustrative example) to provide a relatively flattenedbase segment interposed between the upwardly curving radiused sides.

With this geometry, the part-cylindrical bearing seats 26, 28, definedby offset radii, provide a platform permitting a limited amount of axialrotation and translation. That is, the effect of this special asymmetricarticulating geometry with offset radii is to accommodate asubstantially natural range of anatomic rotational motion on the orderof about plus/minus 5° as viewed in FIG. 4, while at the same timeproviding a limit to extreme rotation motion and restoring a “neutral”position following a rotation motion. In such rotation, the radiusedsides of the insert 20 initially abut the bearing strips 22, 24 of thetop and bottom plates 10, 12 (FIG. 4). Further rotation results in theinsert 20 sliding along the articulating bearing strips 22, 24 of thetop and bottom end plates 10, 12 effectively distracting theintervertebral disc space. This distraction increases loading on theTDI. In turn, the increased loading naturally results in a counteractingforce tending to resist the distraction, forcing the two vertebralbodies back to the “neutral” position.

Thus, this unique TDI articulation geometry functions like the naturaldisc, by limiting axial rotation while permitting normal anatomicflexion-extension and lateral bending motions. No other features such aspositive stops or grooves or additional components such as elastomericmaterials are necessary.

FIGS. 5 and 6 show the implant design in the extreme lateral bending andflexion-extension positions respectively. As can be noted, anatomiccombined lateral bending and flexion-extension range of motion (ROM) arepermitted for the lumbar implant. The total intervertebral height is inthe illustrative embodiment is about 8 mm. For the cervical implant, thedesign permits a higher range of motion—up to about 20° forflexion-extension and lateral bending. The ROM for the proposed lumbarand cervical spines are in accord with those reported by Wilke et al^(xiv, xv) and White and Panjabi^(xvi).

^(xiv) Wilke H J, Kettler A and Claes L, “Are Sheep a ValidBiomechanical Model for Human Spines?”, Spine, 22, 2365-2374, 1997.^(xv) Wilke H J, Krishak S, and Claes L, “Biomechanical Comparison ofCalf and Human Spine”, J. Orthoped. Res., 14, No. 3, 500, 1996. ^(xvi)White A A, and Panjabi M M, “Clinical Biomechanics of the Spine”, 2^(nd)ed.,: J B Lipincott, 1990.

The significant clinical advantages of the design are:

-   -   1. Universal Design for Lumbar and Cervical Applications: the        design, as shown, is suited for a lumbar implant. Suitable        modifications of its dimensions permits its use as a cervical        disc implant. The objective is to offer a range of sizes that        fit the spine in any intervertebral disc space, and without        alteration of that inter-vertebral space regardless of its        natural size or shape.    -   2. Restoration and Preservation of Anatomic Motion: the        objective of the total disc implant (TDI) is to restore        intervertebral space to its natural pre-morbid dimensions and        provide full range of motion including limiting extreme motion        just like in normal spines. A further goal is to prevent        adjacent segment hypermobility. The proposed TDI is more        natural—allowing controlled and limited motion between segments.        In contrast to some current designs, the TDI has a special        bi-convex articulating contour or geometry which permits        “normal” flexion-extension, lateral bending, and limited axial        rotation. This geometry is in sharp contrast to some designs        which use fixed hard stops (e.g., Flexicore), with the potential        for implant loosening, or other discs such as the Charite',        ProDisc and Maverick which have passive stops to limit axial        rotation, and require the annulus to be tightened for optimal        function.    -   3. Minimal End Plate Preparation: by preserving the load bearing        cortical bone and minimizing end plate perforation to expose the        highly vascular cancellous bone, both immediate stability and        long term in-growth is enabled.    -   4. Surgical Technique: the surgical technique for insertion of        these implants is consistent with the established methods of        disc removal, and requires neither specialized instrumentation        nor specialized instrumentation nor specialized surgical        technique. In fact, the technique will be similar to that used        for over a decade for artificial discs. The technique involves        removal of the nucleus pulposus, flattening of the end plates        and leaving most of the annular circumferentially intact. The        anterior or anterior-lateral aspect of the annulus is removed as        needed for TDI placement. Based upon templating the patient        spine and using trial implants, the vertebral bodies are        distracted to a near maximum needed for optimal placement of the        TDI. The fins cut through the end plates and the        osteo-integration surface, which may be domed, is forced into        contact with the cancellous portion of the adjacent vertebral        bodies. Ligamentoxasis is also used to maintain the TDI in        place. The surgical goal is to relieve pain by restoring the        patient's natural spinal anatomy and allowing for some motion        between the diseased vertebral segments, and thereby minimize or        avoid adjacent segment hypermobility. Clinical success will be        defines by reduction or elimination of patient pain, improvement        in function and maintenance of motion with the TDI.    -   5. Extent of Disc Removal: the extent of disc removal can be        determined by the surgeon at the time of surgery and can be        individualized for each patient. As noted above, the end plate        is flattened most of the annulus is left circumferentially        intact. It is contemplated that multiple TDI's with variable        height end plates insert will be provided in order to restore a        unique individual anatomy with a relatively high degree of        precision.    -   6. Elimination of Incorrect Implant Size Selection: in those        implant systems where a drill is used and significant bone is        removed, trials of the implant size must first be made.        Furthermore, regardless of the fit, an implant at least as large        as the space created by the drilling must be utilized,        regardless of the quality of that fit. With the proposed design,        no significant bone is removed, and the correct size implants        are fitted directly to the inter-vertebral space eliminating the        need to guess at the correct implant size before the fact. At        the surgeon's discretion and based on the templating of the        patient's spine, the surgeon will choose the appropriate implant        size in order to restore the patient's spinal anatomy.    -   7. Modular Design: the proposed implant design will be made        available in different standardized A-P depths and M-L widths to        accommodate the physiological range of inter-vertebral space.        The articulating inserts will also be made available in varying        heights within typical the physiological range. This will enable        standardization of the modular implant system over the        lumbar/cervical size ranges.    -   8. Avoidance of Collapse of the Inter-vertebral space: the        implant is made from ceramic material many times stronger than        bone and will not collapse. The implantation technique and TDI        design relies on preservation of the strong vertebral cortex,        which is resistant to compression, thus preventing or minimizing        migration or subsidence of the TDI into the vertebrae. The large        bearing surface area of the implant minimizes the load per unit        area on the insert.    -   9. Revisability: the proposed TDI is an inter-vertebral space        implant and not a “through vertebrae” cross inter-vertebral        space implant. The technique envisioned requires minimal end        plate preparation. Furthermore, the design features multiple 2        mm fins which bite into the adjacent vertebral bone for        stability. It is expected that revision of the implant, should        it become necessary, would be possible with a minimal chance of        iatrogenic destruction of the adjacent vertebrae.    -   10. Self-Stabilizing with Rapid Osteo-integration Capability:        the implant surface is designed to resist dislodgment with        multiple fins assuring immediate anchoring. Long term stability        is provided by rapid osteo-integration into the bio-mimetic        cancellous structured bony ingrowth layer. Loading the porous        layer with osteo-inductive agents can enhance this ingrowth.    -   11. Safety and Versatility: the entire procedure is performed        under direct vision and with complete visualization of the        adjacent vital structures (e.g., organs, neural structures and        blood vessels). The implant also lends itself to a variety of        implantation techniques such as minimally invasive surgery,        anterior, posterior, lateral or extreme lateral approaches.

FIGS. 7-9 show an alternate TDI design for lumbar spine. The designfeatures an upper end plate 30 and a lower end plate 32 formedrespectively with upper and lower surfaces that engage with the adjacentvertebral bodies (not shown). Each end plate 30, 32 includes a solid rim34 substantially circumscribing the respective upper and lower surfaceto rest on the cortex of the adjacent vertebral body. Fixation elementssuch as fins, teeth or pins 36 protrude axially from the respectiveupper and lower surfaces of the end plates 30, 32 to provide anchoringand immediate stability with the adjacent vertebral bodies. These upperand lower surfaces include or are surface-coated each to define a porousin-growth surface 38 to permit and accommodate rapid bone in-growth andosteo-integration for long term stability. A variety of suitable boneingrowth coatings and materials are known to persons skilled in the art.Once again, while illustrative drawings show the in-growth surface 38 tohave a generally planar shape, persons skilled in the art willunderstand that alternative configurations such as a convexly orsimilarly extended surface area contour may be preferred.

The A-P and M-L dimensions of the upper and lower end plates 30, 32 arechosen to suit typical lumbar/cervical spinal body dimensions. Theillustrative end plates 30, 32 further include an anterior to posteriorlordotic taper to better restore the natural curvature of spine, asviewed in FIG. 9.

The articulating lower surface of the upper end plate 30, and thearticulating upper surface of the lower end plate 32 each include aunique bearing component defining a unique bearing surface or contourthat permits a substantially normal range flexion in the A-P directionin combination with extension in the M-L direction, while additionalaccommodating a limited range of axial rotation. These articulatingsurfaces of the upper and lower end plates 30, 32 respectively engageand articulate with each other.

More particularly, the articulating surface of the upper end plate 30comprises a part-cylindrical, downwardly concave bearing component ormember 42 with its axis extending generally perpendicular to the M-Ldirection. The articulating surface of the lower end plate 32 comprisesa similarly sized and shaped, part-cylindrical and upwardly convexelongated bearing component or strip 44 oriented to extend generally inthe A-P direction. Thus, the two bearing surfaces 42, 44 are orientedgenerally on orthogonal axes relative to each other.

Accordingly, the articulating geometry between the upper bearing surface42 on the upper end plate 30, accommodates A-P rotation or flexion (asviewed in FIG. 10), with a preferred range of A-P flexion on the orderof about 12-15°. In a similar manner, the articulating geometry betweenthe lower bearing strip 44 on the lower end plate 30, accommodates M-Lrotation or extension (as viewed in FIG. 11), with a preferred range ofM-L extension on the order of about 12-15° of lateral bending.

In accordance with one primary aspect of the invention, thepart-cylindrical bearing surfaces 42, 44 formed on the upper and lowerend plates 30 and 32 each have a part-cylindrical contour defined byoffset radii, similar to those shown best in FIG. 3. In particular, eachbearing surface 42, 44 is defined by curving sides to be formed as arcsof a circle, but wherein the centers of these arcs are spaced a part orlaterally offset by a small increment to provide a relatively flattenedrotational platform interposed between the curving radiused sides.

With this geometry, the part-cylindrical bearing surfaces 42, 44, andthe flattened rotational platform defined by offset radii, provide aplatform permitting a limited amount of axial rotation and translation.That is, the effect of this special asymmetric articulating geometrywith offset radii is to accommodate a substantially natural range ofanatomic rotational motion on the order of about plus/minus 5° as viewedin FIG. 12, while at the same time providing a limit to extreme rotationmotion and restoring a “neutral” position following a rotation motion.In such rotation, beyond a certain limit imposed by the offset amount,the radiused sides of the curved bearing surface 42 of the top end plate30 slide along the articulating surface 44 of the bottom end plate 32effectively distracting the intervertebral disc space. This distractionincreases axial loading on the TDI. In turn, the increased axial loadingnaturally results in a counteracting force tending to resist thedistraction, forcing the two vertebral bodies back to the “neutral”position.

Thus, this alternate two piece unique TDI articulation geometry as shownin FIGS. 7-9 functions like the natural disc, by limiting axial rotationwhile permitting normal anatomic flexion-extension and lateral bendingmotions. No other features such as positive stops or grooves oradditional components such as elastomeric materials are necessary.Another unique advantage of this design is that it does not require aninsert, thus avoiding any risk of the insert from being dislodged orotherwise impinging on the spine.

The implant design can be flexible enough to permit a higher range ofmotion—up to about 20° for flexion-extension and lateral bending forcervical spine disc replacements. The ROM for the proposed lumbar andcervical spines are in accord with those reported by Wilke et al^(xvii, xviii) and White and Panjabi^(xix).

^(xvii) Wilke H J, Kettler A and Claes L, “Are Sheep a ValidBiomechanical Model for Human Spines?”, Spine, 22, 2365-2374, 1997.^(xviii) Wilke H J, Krishak S, and Claes L, “Biomechanical Comparison ofCalf and Human Spine”, J. Orthoped. Res., 14, No. 3, 500, 1996. ^(xix)White A A, and Panjabi M M, “Clinical Biomechanics of the Spine”, 2^(nd)ed.,: J B Lipincott, 1990.

FIGS. 16-21 illustrate a further alternative preferred form of the TDIof the present invention, based again on principles of maintainingnatural spinal anatomy, restoring function by preserving segmentalmotion, providing immediate implantation stability, withstanding normalspinal loads in a safe and stable manner, and providing relatively rapidand improved osteo-integration between TDI surface and host bone.

FIGS. 16-17 illustrate an upper end plate 60 and a lower end plate 62similar to those shown and described in FIGS. 1-9, but respectivelyincluding convex or domed upper and lower surfaces for engaging adjacentvertebral bodies having an overall size and shape suitable forimplantation into the lumbar spinal region. These domed surfaces aresurface-coated with or otherwise define porous bone in-growth surfaces68 for relatively rapid osteo-integration with porous or cancellousinterior structure of prepared adjacent vertebral bodies. A solid rim 64on each end plate is provided for stable seated engagement with thecircumferential or cortical rim of the prepared adjacent vertebralbodies, so that center loading and potential subsidence is substantiallyeliminated or avoided. Protruding fixation elements 66 such as theillustrative fins are also provided for anchoring the end plates 60, 62,and to provide substantial immediate stability. The illustrativedrawings (FIG. 17) show these fins 66 to have a generally curvedposterior edge and a generally vertical anterior edge suitable foranterior placement. For anterior-lateral placement, a modified fin shapeof generally pyramidal configuration with a triangular base may be used.

As shown and described with respect to the embodiments of FIGS. 1-9, thealternative embodiment of FIGS. 16-21 may incorporate anterior-posteriorand medial-lateral dimensions suitable for specific lumbar or cervicalspinal body dimensions. The end plates 60, 62 have an anterior toposterior lordotic taper (FIG. 17), similar that shown in FIGS. 2 and 9,for better fit and restoration of the natural spinal curvature.

In addition, the articulating surfaces of the end plates 60, 62 have aunique contour that permits flexion-extension and lateral bending whilelimiting extreme rotation. In particular, upper end plate 60 includes adepending, part-cylindrical bearing strip 72 which is elongated in theanterior-posterior (sagital) direction, wherein this bearing strip 72incorporates generally convex opposite end segments separated by acentrally positioned and generally concave segment defining a concavebearing seat 76. By contrast, the lower end plate 62 includes anupwardly projecting, part-cylindrical bearing strip 74 which iselongated along an axis generally orthogonal to the upper bearing strip72. That is, the part-cylindrical lower bearing strip 74 is elongated inthe medial-lateral (coronal) direction, and additionally incorporatesgenerally convex opposite end segments separated by a centrally locatedand generally concave segment defining a concave bearing seat 78.Accordingly, each bearing strip 72, 74 is shaped with generally convexopposite end segments, preferably to expand or taper with increasingdiametric size (FIGS. 16 and 18) from the opposite ends thereof in adirection toward the associated central concave bearing seat 76, 78. Asillustrated in inverted configuration in FIG. 18 with respect to theupper bearing seat 76 formed on the upper bearing strip 72, both concavebearing seats 76, 78 which are also oriented on generally orthogonalaxes relative to each other are desirably formed on offset radii aspreviously shown and described relative to FIGS. 1-9, to define upwardlycurving opposed sides with a relatively flattened base segmentinterposed therebetween.

As viewed in FIGS. 19-21, the above described articulating geometryaccommodates limited relative rotation (FIG. 19) within a limited rangeof about plus/minus 5°, medial-lateral flexion-extension (FIG. 20)within a limited range of up to about 12-15°, and anterior-posteriorlateral bending (FIG. 21) within a limited range of up to about 10-12°.The effect of this articulating geometry including the above-describedconcave surfaces formed on offset radii functions to limit extrememotion and correspondingly to provide an inherent tendency to return toor restore a neutral or substantially centered position between thearticulating components. That is, upon extreme rotation, the combinationof the offset radii for the concave bearing seats 76, 78 and theirengagement on orthogonal axes results in sliding of the upper end plate60 along the lower end plate 62 for distracting the intervertebral discspace. This distraction increases loading in the cranial-caudaldirection on the TDI, which in turn naturally results in a counteractingforce tending to resist the distraction, thereby urging the componentsand the vertebral bodies affixed thereto back toward a neutral position.Thus, the TDI articulation geometry functions like the natural disc, bylimiting axial rotation while permitting normal anatomicflexion-extension and lateral bending motions. No other features such aspositive stops or grooves or additional components such as elastomericmaterials are necessary.

While FIGS. 20 and 21 respectively show the TDI in extremeflexion-extension and extreme lateral bending positions for theillustrative lumbar implant, it is noted the ROM permitted for acervical implant can be varied typically within a wider range of motion.

In summary then, the proposed TDI is a motion preserving prosthetic discfor replacing a damaged disc, which restores anatomic motion andfunction, provides immediate and long term stability and virtuallyeliminates risk from wear particles.

The TDI end plates and/or the insert are constructed from rigid-on-rigidmaterials, such as by use of a selected ceramic material, or a selectedbiocompatible metal, or combinations thereof. In the most preferredform, ultra-low wear bearing materials such as enhanced Si₃N₄ ceramic isused, as shown and described in copending U.S. Ser. No. 10/171,376,filed Jun. 13, 2002, which is incorporated by reference herein.

In particular, for total hip arthroplasty (THA) implant bearingapplications, Si₃N₄ cups/Si₃N₄ heads have demonstrated high safety andreliability in laboratory hip simulator and mechanical tests, and arecost competitive compared to conventional ceramic-on-ceramic bearings.Compared to presently available state-of-the-art ceramics, such Si₃N₄ceramics have 100% higher fracture toughness than alumina and 50% higherfracture toughness than zirconia, a 50% increase in fracture strengthover alumina and no issues with phase transformation or aging likezirconia. They also have very favorable wear performance as determinedover a 3 million cycle test. These properties of Si₃N₄ allowed THAimplants with significantly higher safety and reliability to bemanufactured. Wear performance of these bearings indicates that they arebetter than metal-on-metal bearings by over one order of magnitude, 2orders of magnitude better than metal-PE and 20 times lower thanmetal-XPE bearings. These bearing materials are preferred for use in theTDI of the present invention.

Preferred bio-mimetic, bioactive, cancellous structured ceramics (CSC)for use a porous bone in-growth materials and surfaces are shown anddescribed in copending U.S. Ser. No. 10/137,106, filed Apr. 30, 2002,and entitled RADIOLUCENT BONE GRAFT, and U.S. Ser. No. 10/137,108, filedApr. 30, 2002, and entitled RADIOLUCENT SPINAL FUSION CAGE, both ofwhich are incorporated by reference herein. These CSC ceramics possess[a] high load bearing capability, [b] strong bio-mimetic scaffoldnecessary for ingrowth and rapid integration with host bone, [c] abio-active coating comprising of calcium phosphate (Ca—P), which likehydroxy-apatite (HAP) or tri-calcium phosphate (TCP) is similar to bonemineral, a Ca deficient, carbonate containing apatite similar toCa₁₀(PO₄)₆(OH)₂ and capable of binding to osteo-inductive factors suchas autogenous cells, and [d] good imaging characteristics unlike metals.The fabrication processes to make these bio-mimetic structures have beendeveloped.

The porosity and pore size of these CSC ceramics can be tailored toallow for [a] optimal ingress of vascularization, [b] ease ofcarrying/delivering ex-vivo expanded viable hMSCs within the cancellouscore, and [c] mechanical property match with bone to allow optimalstress transmission.^(xx) The porosity/pore size structure for a loadbearing CSC have been selected using previous reports on the optimalstructure for grafts by Robey and co-workers.^(xxi), Bobyn etal^(xxii,xxiii) and Bloebaum et al^(xxiv). The optimal pore size forachieving bone ingrowth ranged between 100 to 530 μm, with up to 55%porosity. The resultant porous structure 50 of the CSC ceramic closelyresembles the porous structure 52 of trabecular bone, as viewed in FIG.13.

^(xx) S. D. Boden and J H Schimandle, “The Lumbar Spine”, Vol 2, eds. S.W. Wiesel et al, 1996. ^(xxi) Mankani M H, Kuznetsov S A, Fowler B,Kingman A, Robey P G, in Biotech., Bioeng, John Wiley & Sons, Inc. 72:96-107, 2001. ^(xxii) Bobyn J D, Pillar R M, Cameron H U and Weatherly GC, Clin. Orthop., 150, pp.263, 1980. ^(xxiii) Bobyn J D, et al, Clin.Orthop., 149, pp291, 1980. ^(xxiv) Bloebaum R D et al, JBJS, 80,-A,no.4, pp 518, 1981.

These CSCs have been tested in a load bearing sheep condyle defectmodel.^(xxv) using cylindrical CSC plugs (12 mm dia×20 mm long, 70%porous with 500-750 μm pore size) in a sheep condylar defect model(n=6). Extensive in-growth and bone apposition were noted after 12 weeksimplantation. FIGS. 14 and 15 show a typical section, with a CSC plug 54implanted with condylar bone 56. The CSC plugs were coated with auniform amorphous Ca—P coating and pores filled with host bone marrowaspirate. The combination of the Ca—P coating and host osteo-inductivefactors resulted in favorable osteoinductive activity. The implant/boneinterface is depicted in FIG. 15 by reference numeral 58. Referencenumeral 61 refers to a region of bone apposition, and reference numeral63 indicates bone ingrowth.

^(xxv) Lakshminarayanan R., Bireley W., Rao M S, and Khandkar A C,Trans. ORS Mtg., New Orleans, 2003.

Histologic evaluation (Giemsa stain) of thin sections revealed vigorousbone formation both at the implant/host bone interface and within thepores of the scaffold, indicative of the interconnection between pores.At the host bone/CSC implant interface new bone formed at the surface.Complete interconnection of the pores allowed osteoblastic activity topenetrate deeper into the implant and complete vascular penetration wasobserved in the histology evaluation. In addition to primary andsecondary bone, evidence of woven bone was observed. Also, the bone wasviable as detected by osteocytes in the lacunae. Little fibrousencapsulation, presence of macrophage or giant cells was detected.

The extent of bone ingrowth in the coated CSC was significantly highercompared to porous coated Ti plugs of similar dimension in a similarmodel (25% in CSC at 12 weeks. 13% at 12 weeks and 16% at 24weeks)^(xxvi). Comparison of these preliminary results with in-growthdata from the same lab/animal model/implant design/technique indicatethat:

-   -   CSCs at 12 weeks have 200% and 50% higher in-growth when        compared to porous coated Ti plugs at 12 and 24 weeks,        respectively.    -   New bone forms on the surface in CSCs (similar to HAP/TCP        grafts) rather than near the surface (typical for bio-inert        materials such as Ti or alumina), and    -   Penetration depth of new bone is significantly higher.

^(xxvi) Wille B M et al, Trans. ORS Mtg., New Orleans, 2003.

The CSC structure will be used to fabricate the porous in-growth surface18 integrally with the dense bearing surface end plates of the TDIimplant as shown in FIG. 1. Excellent short/long term stability andimaging characteristics will be obtained.

The instant TDI is an ideal prosthetic inter-vertebral disc implant. Thedesign maintains intervertebral anatomy. The TDI design restores spinalsegmental motion and provides a resistance to extreme rotation of thespine as is desired. The biconcave insert 20, with its thicker rim 14will naturally prevent protrusion and resist dislocation. This willminimize risk of pinching nerves or the spinal cord. The ceramic discinsert material offers unprecedented biomechanical safety in carryingand transmitting loads between the vertebrae. The insert is bothbiocompatible and bio-stable: the disc or any of its wear by-products,are highly unlikely to cause adverse tissue reactions. These attributesif demonstrated, will enable the proposed TDI to leapfrog the presentgeneration of disc implants undergoing clinical testing.

A variety of modifications and improvements in and to the improved discimplant of the present invention will be apparent to persons skilled inthe art. For example, it will be recognized and appreciated that theorientations of the bearing strips 22 and 24 on the upper and lower endplates 10, 12 may be reversed, with a corresponding reversal in theorientation of the bearing seats 26, 28 formed in the insert 20.Similarly, it will be appreciated and understood that the offset radiiconcept for forming the bearing seats 26, 28 may also be applied to thebearing strips 22, 24 on the end plates 10, 12. Accordingly, nolimitation on the invention is intended by way of the foregoingdescription and accompanying drawings, except as set forth in theappended claims.

1. A disc implant, comprising: a pair of end plates for affixation toadjacent vertebral bodies; and a bearing component formed respectivelyon each of said end plates and together respectively defining a pair ofelongated bearing surfaces each having a generally part-circular crosssectional shape and at least one of said bearing surfaces furtherincluding laterally spaced-apart, offset radii defined by a generallyflattened base segment interposed between a pair of curved sides, saidbearing surfaces facing each other and extending generally on orthogonalaxes relative to each other.
 2. The disc implant of claim 1 wherein oneof said bearing surfaces extends generally in an anterior-posteriordirection, and the other of said bearing surfaces extends generally in amedial-lateral direction.
 3. The disc implant of claim 1 wherein each ofsaid bearing surfaces has a cross sectional shape defined by laterallyspaced-apart, offset radii defined by a generally flattened base segmentinterposed between a pair of curved sides.
 4. The disc implant of claim1 wherein said bearing surfaces each have an elongated shape definingopposite end segments of generally convex shape separated by a centralsegment defining a generally concave bearing seat, wherein at least oneof said bearing seats has said cross sectional shape defined bylaterally spaced-apart, offset radii defined by a generally flattenedbase segment interposed between a pair of curved sides.
 5. The discimplant of claim 4 wherein said generally concave bearing seat of eachof said bearing surfaces has a cross sectional shape defined bylaterally spaced-apart, offset radii defined by a generally flattenedbase segment interposed between a pair of curved sides.
 6. The discimplant of claim 4 wherein said opposite end segments of each of saidbearing surfaces has a convex shape formed with increasing diametricsize in a direction toward the associated central segment defining saidconcave bearing seat.
 7. The disc implant of claim 1 wherein each ofsaid end plates includes a lordotic taper.
 8. The disc implant of claim1 wherein at least one of said end plates has a tapered thicknessincreasing in a posterior to anterior direction.
 9. The disc implant ofclaim 1 wherein each of said end plates includes means for affixation toadjacent vertebral bodies.
 10. The disc implant of claim 1 wherein eachof said end plates includes a porous bone ingrowth surface foraffixation to adjacent vertebral bodies.
 11. The disc implant of claim10 wherein said porous bone ingrowth surface of each of said end plateshas a generally convex shape for engagement with and affixation toadjacent vertebral bodies.
 12. The disc implant of claim 1 wherein eachof said end plates includes at least one generally axially protrudingfixation element for affixation to adjacent vertebral bodies.
 13. Thedisc implant of claim 1 wherein said pair of bearing surfacesrespectively comprise a ceramic material and a biocompatibie metal. 14.A disc implant, comprising: a pair of end plates for affixation toadjacent vertebral bodies; and a bearing component formed respectivelyon each of said end plates and together respectively defining a pair ofelongated bearing surfaces facing each other and extending generally onorthogonal axes relative to each other; each of said bearing surfacesdefining opposite end segments of generally convex part-circular crosssectional shape separated by a central segment defining a generallyconcave bearing seat, and at least one of said bearing seats beingfurther defined by a generally part-circular cross sectional shapeincluding laterally spaced-apart offset radii defined by a generallyflattened base segment interposed between a pair of curved sides. 15.The disc implant of claim 14 wherein said generally concave bearing seatof each of said bearing surfaces has a cross sectional shape defined bylaterally spaced-apart, offset radii defined by a generally flattenedbase segment interposed between a pair of curved sides.
 16. The discimplant of claim 14 wherein said opposite end segments of each of saidbearing surfaces has a convex shape formed with increasing diametricsize in a direction toward the associated central segment defining saidconcave bearing seat.
 17. The disc implant of claim 14 wherein one ofsaid bearing surfaces extends generally in an anterior-posteriordirection, and the other of said bearing surfaces extends generally in amedial-lateral direction.
 18. The disc implant of claim 14 wherein eachof said end plates includes a lordotic taper.
 19. The disc implant ofclaim 14 wherein at least one of said end plates has a tapered thicknessincreasing in a posterior to anterior direction.
 20. The disc implant ofclaim 14 wherein each of said end plates includes means for affixationto adjacent vertebral bodies.
 21. The disc implant of claim 14 whereineach of said end plates includes a porous bone ingrowth surface foraffixation to adjacent vertebral bodies.
 22. The disc implant of claim21 wherein said porous bone ingrowth surface of each of said end plateshas a generally convex shape for engagement with and affixation toadjacent vertebral bodies.
 23. The disc implant of claim 14 wherein eachof said end plates includes at least one generally axially protrudingfixation element for affixation to adjacent vertebral bodies.
 24. Thedisc implant of claim 14 wherein said bearing surfaces comprise aceramic material.
 25. The disc implant of claim 14 wherein said pair ofbearing surfaces respectively comprise a ceramic material and abiocompatible metal.